In an exemplary semiconductor manufacturing process, an etching process is performed by using a predetermined processing gas while using as a mask a resist film having trenches and holes, which is patterned on the surface of an object to be processed such as a semiconductor wafer (hereinafter referred to as a “wafer”), and then an ashing process is performed to remove the remaining resist film.
For such an ashing process, a method of ashing and removing a resist film (plasma ashing method) by using a processing gas such as O2 gas, CO gas or CO2 gas is known (see, for example, Japanese Patent Laid-open Application Nos. 2003-59911 and 2001-189302). In detail, for example, by introducing O2 gas into a processing chamber and generating plasma while heating a wafer in the processing chamber, the resist film is removed by using active species such as oxygen radicals that are generated when the O2 gas is converted into plasma.
Since the ashing process is performed after the etching process as described above, it is possible to save the time normally required to transfer the wafer to another processing chamber if both etching and ashing processes can be continuously performed in the same processing chamber, resulting in an advantage in which the total process time can be reduced.
However, when a fluorine-containing gas (for example, a CF based processing gas) is used as the processing gas in the etching process, there is a concern that reaction products such as fluorine polymer (for example, CF based polymer) will be deposited on inner walls of the processing chamber. If the ashing process is continuously performed under such a condition, there occurs a phenomenon known as “memory effect” in which the reaction products deposited on the inner walls of the processing chamber, such as the fluorine polymer, become dissociated again and, thus, films on the wafer are etched, so that the quality of the semiconductor device to be formed on the wafer may be degraded. Such a phenomenon occurs when CO2 gas, CO gas or O2 gas is used as the processing gas when performing the ashing process.
Meanwhile, in order to suppress the memory effect, a method of performing the ashing process in two steps is used (see, for example, Japanese Patent Laid-open Application No. H11-145111). First, in the first step, O2 gas is introduced into a processing chamber to remove reaction products without the application of a bias voltage on the wafer, and oxygen plasma is generated, thereby removing the reaction products such as fluorine polymer, deposited on the processing chamber. Thereafter, in the second step, a bias voltage is applied to the wafer and then an ashing gas is introduced into the processing chamber, thereby removing a resist film from the wafer. The process of removing the resist film in two such steps is referred to as “hybrid ashing.”
However, in a case where a layer including a low-k film (hereinafter referred to as a “low-k film”) is formed under a resist film, there is the concern that the low-k film may be damaged when hybrid ashing is simply performed while the low-k film is exposed. In detail, for example, when reaction products such as fluorine polymer, deposited on the inner walls of the processing chamber, are removed in the first step, part of the fluorine is dissociated due to the influence of an oxygen radical generated in the processing chamber and is inserted into the low-k film or an underlying film of the low-k film. In this case, the low-k film or the underlying film of the low-k film is etched, or C (Carbon) atoms contained in the low-k film are separated therefrom and film properties are changed, thereby increasing the dielectric constant.
Although it is also possible to suppress the damage to the low-k film or the underlying film of the low-k film by optimizing the process conditions of the first step, thus reducing the density of oxygen radicals in the processing chamber (see, for example, Japanese Patent Laid-open Application No. 2005-101289), there is a limitation in further suppressing the damage to them.